With an increasing complexity of technical functions occurring on board a motor vehicle, there is an increasing difficulty in localizing faults that occur and assigning them to relevant components. In the context of On-Board-Diagnoses in motor vehicles, as many technical functions as possible should be monitored within the motor vehicle, faults detected and assigned as precisely as possible to defective components, in order then, where a fault is in a component, to bring about a corresponding reaction of the system, e.g., to switch off the defective components.
In fault diagnostics in practice, there is a fundamental problem that directly measuring sensors are available only for some few technical functions, and that therefore the diagnosis of most functions must take place indirectly through monitoring symptoms.
Since, with such fault diagnosis, individual diagnostic functions interfere with each other in many cases, it is often possible, from a single fault symptom, to draw conclusions pointing to more than one cause of the fault. Therefore, distinctions must be made between the actual or true fault and secondary faults. Thus such fault diagnostic devices require a validating device which ascertains whether the component in question on the fault path is actually responsible for the fault symptom or only subject to a secondary influence.
The problem addressed by the device and method according to the present invention thus generally consists in examining fault signals with respect to their meaningfulness and, if possible, subsequently making possible a validation, to obtain certainty in the assertion of the actual cause of the fault.
First, the problem of the mutual dependence of faults is explained in more detail. This problem is based simply in physical or technical reciprocal effects.
There are functions in which the influence occurs only in one direction, master-slave functions, which can be schematically depicted as: EQU A.fwdarw.B
An example is catalytic converter monitoring, which takes place using a lambda probe (oxygen sensor) located behind the catalytic converter. In this context, the correct function of the lambda probe must first be ensured, before it is possible to conclude that a possible fault has occurred in the catalytic converter. In other words, it is assumed that if there is a fault in both diagnoses, that the fault of the lambda probe is responsible for the fault of the catalytic converter. The lambda probe is thus the master and the catalytic converter is the slave.
It is known in the art that the dependent slave function has to wait for a fault-free examination of the superordinate master function. With master-slave functions, a fault of the master function can indeed falsely appear as a fault of the slave function, but not the other way around. Furthermore, usually in the master-slave functions, which influence each other only in one direction, when a fault is detected in the master function, the slave function must be blocked to avoid sequential fault entries in functions in master-slave chains, which in turn depend on the slave function.
In the past, in order to validate the fault, a certain sequence of fault diagnoses has been necessary. In this connection, problems arise in practice. For some diagnostic functions should not or cannot run simultaneously with certain operating functions. It is true that if the fault diagnostic device has to wait each time for the presence of a certain operating function, then the fault diagnosis, under certain circumstances, would proceed very slowly.
The situation is even more complex where functions that influence each other in both directions, i.e., with cyclical master-slave dependencies, such as can occur in the simplest case thus: EQU A.fwdarw.B.fwdarw.A or A{character pullout}B
If the diagnosis of function A yields a fault, the diagnosis of function B is necessary to validate the fault in A. However, for validating B, A is necessary in turn, which makes it impossible, from these functions A and B, to determine the true fault and the secondary fault. This is a simple deadlock situation. An example of a deadlock situation is the examination of functions of the secondary air valve and of the tightness of the suction line by measuring the air ratio as measuring parameter.
A deadlock situation, or, more succinctly, a deadlock, always results when two mutually influencing functions both indicate a fault and block each other so that neither can the fault subsequently be confirmed nor its removal be detected.
A deadlock situation can also arise from more than two functions, i.e., if the following sort of dependence obtains: EQU A.fwdarw.B.fwdarw.C.fwdarw.A
It may be referred to as a general or indirect deadlock if each one of the functions A, B, C indicates a fault.
In the past, for validating such faults, one direction of simple or general deadlocks was completely neglected, and indeed usually in accordance with the probability of occurrence of the fault in question. The closed cycle was broken where the smallest probability of a fault was expected.
In the above-described conventional design, the fact that it cannot systematically take account of deadlocks has proven to be disadvantageous. Therefore, a fault diagnosis system is required, which makes possible any and all sequences of fault diagnoses at a correspondingly high speed and in which, after the event, the mutual dependance of faults can accordingly be taken account of.